The Thermal Decomposition of N-Arylcarbamates of t-Alcohols. 2. Effects of Solvent, Temperature, and Substituents

Abstract

The influence of solvent, ring substituents, and tertiary carbon atom substituents on the decomposition at 423–473 K of some N-arylcarbamates of tertiary alcohols (p-XC6H4NHCO2C(CH3)2R, where X = CH3, H, Cl, NO2, R=C≡CH, C2H5, CH=CH2) has been investigated. In dodecane, diphenyl ether, acetophenone, nitrobenzene, and decanol, the reactions were first order yielding chiefly carbon dioxide, amine, and olefin. t-Pentyl carbamates yielded the olefins 2-methyl-1-butene and 2-methyl-2-butene in a 3:1 mixture. Solvent effects are reflected in variations in enthalpies and entropies of activation, but effects on the actual values of the specific rate constants are small. Substituent effects are also small, with changes in R from C≡CH to CH=CH2 producing only a tenfold rate increase. Variations in X yield Hammett σρ plots with slopes of about 0.50. These results are consistent with the charge separated cyclic transition state proposed before (1). In ethylene glycol, increased reaction rates and 1:1 yields of the olefins from the t-pentyl carbamates indicate a mechanism with greater carbonium ion character.

The Effect of Outer‐Sphere Acidity on Chemical Reactivity in a Synthetic Heterogeneous Base Catalyst

“…These results show that the acidic outer sphere in 3 reduces the activation barrier for thermolysis by about 4 kcal mol −1 relative to that of 4 . These activation energies are consistent with previously reported activation energies for the olefin‐formation step in the thermolysis mechanism, which is thought to be rate‐limiting 15. Similar effects have been observed with imprinted materials derived from 2 .…”

“…Although mechanistically more complex than the proton transfer, the thermolysis of carbamates 3 and 4 is expected to be accelerated by the presence of accessible silanol groups in the outer sphere because of the known acid lability of such carbamates. A related phenomenon has been demonstrated in homogeneous thermolysis of N ‐arylcarbamates of tertiary alcohols, for which the activation energy for thermolysis decreases by approximately 6 kcal mol −1 between decane and the protic solvent decanol 15…”

Acid−Base Bifunctional and Dielectric Outer-Sphere Effects in Heterogeneous Catalysis: A Comparative Investigation of Model Primary Amine Catalysts

“…Thermolytic carbamate decomposition in materials 1, 2, and 3 proceeds via a charge-separated transition state involving protonation of the carbonyl oxygen. 41 Based on this mechanism, both the dielectric environment and the presence of an acidic silanol functionality in the outer sphere should affect thermolysis kinetics. Figure 1 difference caused by dielectric effects is apparent in the slightly lower thermolysis temperature of 3 versus 2.…”

Carbamates and 2-oxazolidinones from tertiary alcohols and isocyanates

“…Their lower deco~nposition temperatures and ease of preparation offer advantages over acetates and xanthates which have been used previously (8). A kinetic study of the thermal degradation of the monocarbamates has already been reported (9,10).…”